As well as the symmetry tools in Cheetah 3D, it would be great to have a symmetric mode (so that any operation performed on the positive side of the X axis was reflected on the negative side and vice versa.

This would allow lower polygon counts (because you wouldn't be forced to place an often superfluous seam down the middle of the model) and would be more versatile for a lot of purposes.

I often had the problem in cheetah, that the two halfs were not welded correctly (soft models). for example an egg. there was always a "sharp" edge
all around the shapes outer symmetrical axxis, had to do "micro operations" to correct it

Here is the difference between the modifier and the symm tool/ability that I would like to have and I think pretty much what Pod wants too.

Assuming the mesh is symmetical in the first place. For example, if one starts with a cube or sphere and make it editable. Let's use the z-axis plane as the plane dividing the cube in half. What I want to do is to turn on symmetry along the z-axis, select the "right" side face of the cube and move (or extrude, or perform some function) that face along the x-axis. The symmetry function would move (extrude, etc...) the opposite face and move it along the x-axis, in the opposite direction.

Yes, it's similiar to the modifier, but this allows one to work a model that doesn't/won't have a seam running down it's middle. AND it will allow the user a way to work on both sides of a mesh at the same time at the fine-tuning part of the workflow. Right now, the user would either have to the same actions on both sides or must cut the mesh in half, do the work and mirror it.